Another way to understand Chart is a report with Charts (not a specific Chart), so Charlie want to put Cpk on the report but don't know how to calculate.
Charlie, in your report, I can't see your specification limits,
without them is not possible to calculate.
Cpk = min (X_Average-LSL, USL-X_Average)/(3Sigma)
As you didn't ask for Cp, looks like your process may have only one spec limit, so use the only one if it is the case.
Extracted from your formulae
3Sigma = 0.58*R_Average.